The present embodiments relate to a shim coil device for a magnetic resonance device.
Magnetic resonance imaging is a known, established imaging technique that is used in the medical engineering area. In order to obtain images useful for diagnostic assessment, the aim has been to obtain a signal-to-noise ratio (SNR) that is as high as possible. The use of local coils for this purpose is known. For example, antenna systems arranged in the immediate vicinity, on (anterior) or below (posterior) the patient, have been used. During the recording of magnetic resonance image data, the excited cores induce a voltage in the individual antennas or conductor loops of the local coils. The induced voltage is amplified with a low noise preamplifier (LNA) and forwarded via a cable to a receive device. The receive device further processes the magnetic resonance image data.
A further improvement in the signal-to-noise ratio (e.g., for high-resolution magnetic resonance images) is the aim of high-field systems. The basic magnetic field of high-field systems has a strength of, for example, 1.5 T to 12 T or more.
For all magnetic resonance imaging purposes, homogeneity of the basic magnetic field (B0) is of importance. If variations in the homogeneity of the imaging volume are too great, artifacts or distortions may arise. Also, specific applications such as fat saturation may no longer function. Fat saturation (e.g., fat set) is an imaging technique, in which the frequency shift of protons bound to fat or fatty materials is used. A saturation pulse at the fat frequency is used as a send pulse to exclude the frequency of signals originating from fatty tissue. Since the difference between the proton frequency in water and the proton frequency in fat is very small (e.g., equivalent to a few ppm of the basic field), this imaging technique is heavily dependent on the spatial homogeneity of the basic magnetic field. In such cases, a homogeneity of up to 0.5 ppm may be reached over imaging volumes of approximately 30×30 cm.
In this context, the influence of the object to be recorded (e.g., a patient) on the homogeneity of the basic magnetic field is not to be ignored. Thus, in the region of the nape of the neck of the patient, the body tissue is significantly susceptible to inhomogeneous body tissue distribution. Because of inhomogeneous body tissue distribution, distortions of the basic magnetic field may occur. To rectify these distortions, shim coils are used. In such cases, the number of different shim coils, their arrangement, and activation only allows a restricted number of degrees of freedom to compensate for inhomogeneities of the basic magnetic field. These inhomogeneities are generated by a mostly superconducting basic field magnet, by shim currents, and by corresponding shim magnetic fields, which are generated by conventional coils (e.g., copper coils). The options or degrees of freedom provided by the shim coils are not sufficient to compensate for inhomogeneities in many already known magnetic resonance devices (e.g., in the neck vertebrae area). A great jump in susceptibility arises precisely in the neck vertebrae area as a result of the transition from thorax to the neck/head.
However, homogeneity is not just disturbed by the presence of an object or patient to be detected, but may also be disturbed by the properties of the magnet system itself. For example, inhomogenities to be corrected may occur at the edge of the homogeneity volume.
German patent application DE 10 2011 077 724 proposes to compensate for sharply varying local magnetic fields within a magnetic resonance device by using shim coils as part of a local coil, so the shim coils are located close to the region of interest (roi) in the patient to be examined.
A problem with the use of magnetic resonance imaging in the field of the fat saturation techniques is inadequate fat saturation. Fatty tissue, in the area of the nape of the neck, for example, is still shown as brightly illuminated in an image, even though fat saturation techniques are designed to hide the fatty tissue in the image. This effect occurs because the fatty tissue in this area does not have the expected resonance frequency due to the local B0 variation. The saturation pulse, which should fully activate the spins of the fatty tissue, does not reach the spins, since the resonant frequency is different. A shim coil provided as part of the local coil may help to alleviate this issue.
An issue with this approach is that the shim coil is to be decoupled from the gradient field, since there may be no disruption of the shim activity and, therefore, also the shimming effect of the shim coil. Lowpass filtering may be used, since the gradient fields are low-frequency. However, the use of ferrite filters within the local coil and the use of ferrite filters of suitable size in the magnetic field is not possible. Ferrite-free filters are not used in a local coil because of the volume. Without such lowpass filtering, however, the current induced in the shim coil leads to a distortion of the gradient field or a distortion of the shim field generated by the local shim coil. Since the position in the k-space represents the time integral over the gradients, even small disturbances may accumulate rapidly over the sequence runtime. A further peculiarity of shim coils integrated into the local coil is that since the local coil itself is able to be moved directly or via the movement of the patient support, the decoupling is independent of the position of the shim coil integrated into the local coil in the longitudinal direction of the patient chamber (e.g., z-direction) and may have been realized independently of a position of the shim coil.